1. Field of the Invention
The present invention relates to a CDMA base station system for controlling the directivity of a plurality of antennas as a whole and transmitting a spread signal to a mobile station system with the transmission power depending on the mobile station system, and more particularly to a CDMA base station system having such transmission directivity characteristics as to reduce interference given to a relatively interference-susceptible mobile station system to a level lower than a conventional level.
2. Description of the Related Art
Efforts are being made to study DS-CDMA systems as radio transmission systems according to the IMT-2000 standardization process. In a radio communication system based on DS-CDMA principles, different spreading codes are assigned to respective mobile station systems. In a CDMA base station system, a received signal is despread using a spreading code assigned to a certain mobile station system for thereby separating the signal destined for the mobile station system from the received signal, and a signal spread using a spreading code assigned to a certain mobile station system is transmitted to the mobile station system.
The radio communication system based on DS-CDMA principles allows a plurality of mobile station systems to share one frequency band by using the above spreading codes. However, since the same frequency band is shared by plural mobile station systems, a spread signal transmitted to and from a certain mobile station system serves as an interference signal for a spread signal transmitted to and from another mobile station system. Therefore, if a multirate service is provided using a DS-CDMA system, then it has been proposed to provide a CDMA base station system with an adaptive antenna array for interference removal. The proposed elimination of interference with an adaptive antenna array will be described below.
The multirate service provided as a DC-CDMA feature is a service which allows the user to obtain a plurality of communication rates. The multirate service can offer voice communications similar to conventional voice communications, and can also transmit high-bit-rate data and image data using a high communication rate.
The adaptive antenna array comprises a plurality of antennas whose overall directivity can be controlled. Specifically, a reception weight or a transmission weight is given to each of the antennas for controlling the directivity upon reception (reception directivity) and the directivity upon transmission (transmission directivity).
In the multirate service, generally, the power level of a signal is greater as the communication rate of the signal is higher. Therefore, when a signal having a relatively high communication rate (high-rate user signal) and a signal having a relatively low communication rate (low-rate user signal) are simultaneously transmitted from a plurality of mobile station systems, the high-rate user signal interferes with the low-rate user signal. To solve this problem, it has been proposed to provide a CDMA base station system with an adaptive antenna array for interference removal upon signal reception. For example, prior proposed attempts are disclosed in xe2x80x9cIEICE Trans. Fundamentals, vol. E80-A, no. 12, pp. 2445-2454, December 1997, S. Tanaka, M. Sawahishi, and F. Adachixe2x80x9d, and xe2x80x9cIEEE VTC99-Conf. Rec., vol. 1, pp. 243-248, May 1999, S. Tanaka, A. Harada, M. Sawahishi, and F. Adachixe2x80x9d.
For interference removal upon signal reception by the CDMA base station system, the reception weights of the adaptive array antenna are controlled to achieve a reception directivity for preferentially attenuating a reception interference signal having a large power level. For example, if a high-rate user signal and a low-rate user signal are present as interference signals other than a signal that is desired to be received, then the high-rate user signal as an interference signal having a relatively large power level is preferentially attenuated, thus improving signal reception characteristics.
It has also been proposed to improve signal transmission characteristics by controlling the transmission weights of an adaptive antenna array with a CDMA base station system not only for uplink communications from mobile station systems to the CDMA base station system but also for downlink communications from the CDMA base station system to mobile station systems. One proposal is disclosed in xe2x80x9cIEICE Technical Report, RCS99-18, May 1999, Harada, Tanaka, Sawahashi, and Adachixe2x80x9d, for example.
Specifically, there has been studied a CDMA base station system for controlling the transmission directivity of an adaptive array antenna using transmission weights based on the reception weights that were obtained for interference removal upon signal reception. The transmission weights based on the reception weights are weights obtained by effecting a calibration process on the reception weights. The calibration process serves to remove effects due to the difference per antenna between the complex amplitude characteristics of an RF receiver and the complex amplitude characteristics of an RF transmitter in the CDMA base station system.
If the transmission directivity of an adaptive array antenna is controlled using transmission weights based on the reception weights, as described above, then since more priority is given to the reduction of interference imposed on a mobile station system (high-rate user) which transmits and sends a high-rate user signal, i.e., a signal having a relatively large power level, than a mobile station system (low-rate user) which transmits and sends a low-rate user signal, i.e., a signal having a relatively small power level, the signal reception characteristics of the low-rate user that is more susceptible to interference than the high-rate user are largely deteriorated.
Stated otherwise, although the transmission directivity of the adaptive array antenna actually needs to be controlled in order to preferentially lower the antenna gain in the direction of the low-rate user for the downlink, it has heretofore been customary to preferentially use the freedom of the adaptive antenna array so as to lower the antenna gain in a multipath direction of the high-rate user signal, for example. Consequently, it is highly difficult to lower interference imposed on the low-rate user. This difficulty manifests itself particularly if many low- and high-rate users are present compared with the freedom of the adaptive antenna array.
One way of determining the reception directivity of the adaptive array antenna with the CDMA base station system has been to employ an MMSE (Minimum Mean Square Errors) control process for determining reception weights in a manner to minimize the power levels of received signals other than a desired received signal. If the desired received signal arrives at the CDMA base station system via a plurality of paths in a multipath environment, then a signal that is received at an earliest time (leading wave) among multipath signals received by the CDMA base station system via the plural paths is selected as a reference signal, and reception weights are determined in order to minimize the power levels of received signals other than the reference signal.
However, the leading wave among the signals received by the CDMA base station system via the plural paths may not necessarily arrive at the CDMA base station system via the path whose power loss is minimum. Accordingly, the determined reception weights may not be optimum.
Specific examples of reception adaptive array antenna devices are revealed in xe2x80x9cIEEE Trans. Information Theory, vol. IT-32, no. 2, pp. 195-219, March 1986, J. W. Modestino and V. M. Eyubogluxe2x80x9d and xe2x80x9cIEICE Technical Report, A.P97-146, November 1997, Kazuhiko Fukawaxe2x80x9d, for example. In the disclosed reception adaptive array antenna devices, the position (time) of a multipath wave selected as a reference signal is fixed in advance, failing to employ reception weights that are optimum at all times.
In the above various conventional arrangements, the power of the signal transmitted from the CDMA base station system to the mobile station system differs from communication rate to communication rate. The shortcomings described above also arise when the transmission power is controlled depending on the distance by which a CDMA base station system is spaced from a mobile station system, as described below.
The CDMA base station system which performs transmission power control transmits a signal to the mobile station system with a power that is greater as the distance from the CDMA base station system to the mobile station system is larger. Therefore, all mobile station systems are capable of receiving signals from the CDMA base station system at the same power level regardless of the distances by which the mobile station systems are spaced from the CDMA base station system. If the mobile station systems perform similar transmission power control, the CDMA base station system can receive signals from the mobile station systems at the same power level.
If the CDMA base station system can receive signals from the mobile station systems at the same power level, then when the CDMA base station system controls the transmission directivity of the adaptive array antenna using transmission weights based on the reception weights used for signal reception, the transmission directivity control process does not preferentially use the freedom of the adaptive array antenna so as to preferentially lower the interference imposed on either one of the mobile station systems.
However, because a mobile station system which is spaced from the CDMA base station system by a relatively small distance, i.e., a short-distance user, is affected more greatly by the interference due to a signal transmitted from the CDMA base station system to another mobile station system than a mobile station system which is spaced from the CDMA base station system by a relatively large distance, i.e., a long-distance user, the above transmission directivity control process tends to greatly deteriorate the reception characteristics of the short-distance user.
As described above, inasmuch as the conventional CDMA base station system controls the transmission directivity of the adaptive array antenna using transmission weights based on the reception weights that preferentially lower a reception interference signal having a large power level, the interference imposed on relatively interference-susceptible mobile station systems, e.g., low-rate users or short-distance users, becomes large, largely deteriorating the reception characteristics of those relatively interference-susceptible mobile station systems.
When the conventional CDMA base station system carries out the MMSE control process in a multipath environment, for example, since the leading wave is selected as the reference signal, there are occasions where optimum weights may not be determined.
It is an object of the present invention to provide a CDMA base station system which receives spread signals transmitted from mobile station systems with a plurality of antennas and gives transmission weights respectively to the antennas to control the overall directivity of the antennas for preferentially lowering the interference imposed on relatively interference-susceptible mobile station systems when transmitting a spread signal to a mobile station system with the transmission power that corresponds to the mobile station system.
Another object of the present invention is to provide a CDMA base station system which is capable of employing optimum transmission weights by selecting an optimum reference signal at all times in controlling the transmission directivity according to an MMSE control process in a multipath environment where a spread signal transmitted from a mobile station system is received via a plurality of paths.
To achieve the above objects, a CDMA base station system according to an aspect of the present invention receives a spread signal transmitted from a mobile station system with a plurality of antennas, imparts transmission weights to the respective antennas for controlling the overall directivity of the antennas, and transmits a spread signal to a mobile station system as a transmission partner with a transmission power corresponding to the mobile station system as a transmission partner. The CDMA base station system transmits the spread signal using transmission weights that are determined as described below. It is assumed that the transmission power corresponding to the mobile station system as a transmission partner is equal to or greater than a predetermined threshold value, i.e., a mobile station system corresponding to the same transmission power as the transmission power corresponding to the mobile station system as a transmission partner is relatively resistant to interference.
In the above CDMA base station system, reference signal detecting means detects, as a reference signal, a signal received from a mobile station system as a transmission partner corresponding to a transmission power equal to or higher than a predetermined threshold value, compensation-free signal detecting means detects, as compensation-free signals, signals received from other mobile station systems corresponding to transmission powers which are higher than the transmission power corresponding to the mobile station system as a transmission partner, and transmission means calculates transmission weights based on reception weights of the antennas in order to reduce the reception level of another signal than the detected reference signal and the detected compensation-free signals and increase the reception level of the reference signal, and transmits a spread signal to the mobile station system as a transmission partner using the calculated transmission weights.
Using the above transmission weights, a signal can be transmitted at a large power level to the mobile station system as a transmission partner, and can be transmitted at a small power level to a mobile station system corresponding to a transmission power smaller than the transmission power corresponding to the mobile station system as a transmission partner. Therefore, the interference imposed on a mobile station system corresponding to a relatively small transmission power, i.e., a relatively interference-susceptible mobile station system, can preferentially be reduced.
The transmission weights of the respective antennas comprise adjusted values of amplitude and phase of a spread signal that is to be transmitted from the antennas. However, the transmission weights may not necessarily be adjusted values of both amplitude and phase, but may be adjusted values of amplitude only or adjusted values of phase adjusted values of both amplitude and phase only. The CDMA base station system imparts those transmission weights to the respective antennas for thereby allowing the overall antennas to achieve a transmission directivity depending on the transmission weights.
The transmission weights may be calculated directly without calculating reception weights, or may be calculated based on reception weights after the reception weights have been calculated.
If the transmission weights are calculated based on reception weights after the reception weights have been calculated, then the reception weights may comprise the above adjusted values, as with the transmission weights. Since the reception weights represent a concept for calculating transmission weights, any reception weights may be used for actual signal reception. Usually, the reception weights according to the present invention are not used for actual signal reception, but different reception weights preferred for actual signal reception are used for actual signal reception.
According to another aspect of the present invention, a CDMA base station system receives a spread signal transmitted from a mobile station system with a plurality of antennas, imparts transmission weights to the respective antennas for controlling the overall directivity of the antennas, and transmits a spread signal to a mobile station system as a transmission partner with a transmission power corresponding to the mobile station system as a transmission partner. The CDMA base station system transmits the spread signal using transmission weights that are determined as described below. It is assumed that the transmission power corresponding to the mobile station system as a transmission partner is smaller than a predetermined threshold value, i.e., a mobile station system corresponding to the same transmission power as the transmission power corresponding to the mobile station system as a transmission partner is relatively susceptible to interference.
In the above CDMA base station system, reference signal detecting means detects, as a reference signal, a signal received from a mobile station system as a transmission partner corresponding to a transmission power smaller than a predetermined threshold value, compensation-free signal detecting means detects, as compensation-free signals, signals received from other mobile station systems corresponding to transmission powers which are higher than the transmission power corresponding to the mobile station system as a transmission partner, and transmission means calculates transmission weights based on reception weights of the antennas in order to reduce the reception level of another signal than the detected reference signal and the detected compensation-free signals and increase the reception level of the reference signal, and transmits a spread signal to the mobile station system as a transmission partner using the calculated transmission weights.
Using the above transmission weights, a signal can be transmitted at a large power level to the mobile station system as a transmission partner, and can be transmitted at a small power level to a mobile station system corresponding to a transmission power equal to or smaller than the transmission power corresponding to the mobile station system as a transmission partner. Therefore, the interference imposed on a mobile station system corresponding to a relatively small transmission power, i.e., a relatively interference-susceptible mobile station system, can preferentially be reduced.
The transmission weights and the reception weights are the same as those described above.
The CDMA base station system is used for communications in a multipath environment where a spread signal transmitted from a mobile station system as a transmission partner is received via a plurality of paths. The reference signal detecting means comprises means for detecting, as a reference signal, a signal whose reception power is maximum among signals received from the mobile station system as a transmission partner is received via the paths, and the compensation-free signal detecting means comprises means for detecting signals other than the signal detected as a reference signal among signals received via the paths from the mobile station system as a transmission partner, as being included in the compensation-free signals.
Since a signal whose reception power is maximum among multipath signals received via the plural paths, i.e., a signal received via the best path whose power loss is minimum, is selected as a reference signal, it is possible to calculate the best transmission weights at all times for signal transmission.
The CDMA base station system is used for communications in a multipath environment where a spread signal transmitted from a mobile station system as a reception partner is received via a plurality of paths. In the CDMA base station system, reception reference signal detecting means detects, as a reception reference signal, a signal whose reception power is maximum among signals received from a mobile station system as a reception partner is received via the paths, and reception means calculates reception weights of the respective antennas in order to reduce the reception level of another signal than the detected reception reference signal and increase the reception level of the reception reference signal, and receives a spread signal from the mobile station system as a reception partner using the calculated reception weights. The reception weights comprise reception weights that are used for actual signal reception.
Using the above reception weights, a signal can be received at a large reception level from the mobile station system as a reception partner. For receiving a signal from another mobile station system at a small reception level, a signal whose reception level is maximum among multipath signals received from the mobile station system as a reception partner via the plural paths is selected as a reception reference signal. Consequently, as with the above transmission weights, best reception weights can be calculated at all times for signal reception.